pdfGetLayerOrderNumChildren

PDF files with layers contain a tree structure that specifies the way
layers are intended to be displayed to the user. Each node in the tree
has either a name or a layer. Named nodes are used to provide a
hierarchical structure for the layers, with labels to help the
user. Each node also optionally has a list of child nodes.

This function returns the number of children that the specified layer
order tree node has.